The X-15 was not the first rocket-powered aircraft, but it is probably the best one ever built and flown. Before the first X-15 took flight in the late 1950s, the fastest speed airplanes had reached was Mach 3. The X-15 doubled that and also went on to fly into space more than a dozen times.
The US Air Force and NASA developed the X-15 to better understand flight under extreme conditions, including reentry through the Earth’s atmosphere. Yet more than half a century later, the exceptional plane still holds the world record for speed by a piloted, powered aircraft after William Knight flew the vehicle at Mach 6.70 in 1967.
The X-15 program also boasts an exclusive club of pilots—only a dozen aviators can claim to have flown the aircraft, which made 199 flights in total. Before he landed on the Moon, Neil Armstrong flew seven X-15 missions between 1960 and 1962. The movie First Man vividly depicts one of these flights.
On the night before a flight, technicians would mate the X-15 to the B-52 aircraft, pressurizing the plane’s fuel tanks and preparing it for flight. The pilot would arrive shortly after sunrise to don a pressure suit and ensure the integrity of its seals. About 45 minutes before the carrier aircraft took off, he would climb into the X-15 cockpit and connect to life support.
Then, the wheels would roll, and the B-52 would take off for its ride up to the launch location and an altitude of about 46,000 feet. This would take a little more than an hour.
At the drop-off point, the pilot would hit the launch button to release the X-15 from its carrier aircraft. As soon as it was clear of the B-52, he would grab the throttle as quickly as possible to open the propellant lines.
The liquid-fueled XLR99-RM-2 rocket engine, built by Reaction Motors, had 70,400 pounds of thrust. This was a powerful engine six decades ago, and even today it has about two-thirds of the thrust that Blue Origin’s New Shepard rocket uses to send a six-person capsule into space. At the time of launch, the engine was in “igniter idle” status, with enough propellant mixture in the engine chamber to immediately ignite when propellant began flowing from the main fuel tanks. Because the X-15 weighed about 30,000 pounds fully loaded with fuel, the initial acceleration was about 2Gs.
The engine would burn for about 90 seconds, and the force pushing against the pilot would build to about 4Gs, making it more difficult to reach upward, toward the engine panel. This initial part of the flight required absolute attention, because any course corrections had to be made early, before the X-15 climbed out of the lower atmosphere. Once on a ballistic trajectory, there wasn’t much that could be done to alter the flight.
The X-15 didn’t have brakes or the landing gear of a traditional airplane, rather it had a front wheel and skids at the back. For nominal flights, the landing took place at a large, dry lake bed, 21km long. Pilots flying the X-15 had practiced landing here and other similar, flat lake beds, which had the consistency of a clay tennis court, hundreds of times in an F-104 airplane.
The X-15 Rocket Plane – NASA Aerospace Documentary
I saw it at the National Air and Space Museum in Washington DC.